This invention relates to a method of manufacturing a low-core-loss grain oriented electrical steel sheet suitable for use as a material of cores of electrical apparatuses, e.g., transformers.
Grain oriented electrical steel sheets are mainly used as transformer core materials, and it is necessary for such materials to have good magnetic properties. It is particularly important to reduce the energy loss, i.e., core loss of grain oriented electrical steel sheets used as core materials.
Various attempts have been made to reduce the core loss, either by setting the orientation of crystals to the (110) [001] orientation highly uniformly, or increasing the electrical resistance of the steel sheet by increasing the Si content thereof, or reducing impurities, or reducing the sheet thickness, or using other techniques or methods.
Such conventional methods have enabled manufacture of steel sheets to be improved to some extent, that is, to achieve a core loss W.sub.17/50 of 0.9 W/kg or less (at a magnetic flux density of 1.7 T, 50 Hz) with respect to sheet thicknesses not greater than 0.23 mm.
However, further substantial improvements in core loss cannot be expected as long as only metallurgical methods are used.
Recently, various methods of artificial domain refining have been tried as a means for achieving a substantial reduction in core loss. Some of them have been applied to industrial processes. For example, a method such the one described in Japanese Patent Publication No.57-2252 in which the surface of a steel sheet processed by final texture annealing is irradiated with a laser beam, is known.
This method can achieve a substantial reduction in core loss and, hence, manufacture of a steel sheet is possible having a thickness of 0.23mm and a core loss of W.sub.17/50 of 0.85 W/kg or less (at a magnetic flux density of 1.7 T, 50 Hz).
This method, however, entails a drawback in that if a heat treatment such as stress relief annealing including heating at 600.degree. C. or higher is effected after laser irradiation, the laser irradiation effect is lost. Therefore the sheet is not suitable for the cores of wound core transformers which require stress relief annealing.
Japanese Patent Publication Nos. 62-54873 and 62-53579 disclose methods of manufacturing low-core-loss grain oriented electrical steel sheets which can be used for wound core transformers. In the method disclosed in Japanese Patent Publication No. 62-54873, thread-like grooves are locally formed in a final texture annealed steel sheet by locally removing the insulating layer on the steel sheet by laser or by mechanical means and effecting etching by an acid solution on the portions from which the insulating layer has been removed, or by scribing the steel directly and mechanically with a knife or the like, and forming a phosphate tension-applying coating so as to fill the grooves. In the method disclosed in Japanese Patent Publication No. 62-53579, grooves having a depth of 5 .mu.m or greater are formed in a final texture annealed steel sheet with a load of 90 to 220 kg/mm.sup.2, and the steel sheet is thereafter heat-treated at 750.degree. C. or higher.
These methods of forming grooves, however, are disadvantageous. In the case of Japanese Patent Publication No. 62-54873, it is difficult to remove the coating with stability because of differences in the coating thickness and the light absorption coefficient and, hence, to form grooves uniformly. In particular, where direct mechanical scribing is effected, burring occurs along the periphery of each groove, resulting in a reduction in space factor. In the case of Japanese Patent Publication No. 62-53579, it is difficult to adjust the load for obtaining grooves having a constant depth. Where grooves are formed in a final texture annealed steel sheet as in these methods, the coating is damaged by the formation of the grooves to an extent such that it is necessary for the steel sheet to be coated with the insulating material again, resulting in a reduction in space factor and an increase in manufacturing cost.
Japanese Patents Laid-Open Nos. 59-197520 and 63-42332 disclose methods arranged to solve these problems: one in which thread-like grooves are formed in a cold-rolled steel sheet having a final gauge by knife edges, laser or other means, and one which is based on photoetching or electrolytic etching using a stencil.
The method disclosed in Japanese Patent Laid-Open No. 59-197520 is free of any need for re-coating but still entails a need to remove burring formed along the periphery of each groove. In the case of the method disclosed in Japanese Patent Laid-Open No. 63-42332, there is difficulty in uniformly maintaining, for photoetching, the state of exposure to ultraviolet light through the mask and the state of etching of the exposed portion immersed in a developer with respect to the whole of the coil, and the problem of difficulty in uniformly forming grooves by electrolytic etching using a stencil because of spreading of the electrolytic solution. In particular, when the thickness of the steel sheet is small, 0.27 mm or less, there are large variations in magnetic properties, and these properties cannot always be stabilized.